1. Field of the Invention
The present invention relates to a production process of a polymerized toner used in development for copying machines, facsimiles, printers and the like by an electrophotographic system.
2. Description of the Related Art
In recent years, image forming apparatus using an electrophotographic system, such as copying machines and printers, have rapidly trended toward formation of full-color images. In color printing by the electrophotographic system, even high-definition images including photographs are printed, so that there is a particular demand for printing high in resolution and good in color reproducibility. Therefore, high-quality color toners, which can meet requirements for such high printing property, are required.
In order to meet the above requirements, it is known that the use of a spherical toner improves transferability and developability and is suitable for high resolution. Since a polymerization process such as a suspension polymerization process or emulsion polymerization process can efficiently produce a spherical toner, it is suitable for use as a production process thereof. A toner produced by such a polymerization process is called “a polymerized toner”.
In the polymerization process, a polymerizable monomer composition comprising a polymerizable monomer and a colorant is generally suspended or emulsified in an aqueous dispersion medium to form fine droplets, and the fine droplets are then polymerized, thereby producing colored polymer particles. The colored polymer particles are used as polymerized toner particles. In order to obtain a polymerized toner capable of conducting printing high in resolution and good in color reproducibility, it is necessary to finely and uniformly disperse the colorant in the polymerizable monomer composition. In order to finely and uniformly disperse the colorant, there is a method in which high shearing force is applied to a mixture containing the polymerizable monomer and the colorant to stir the mixture, thereby dispersing the colorant. As a dispersing machine, various kinds of media type dispersing machines containing media particles are generally used.
For example, Japanese Patent Application Laid-Open No. 6-75429 discloses a production process of a polymerized toner, which comprises the steps of dispersing a colorant in a monomer composition containing at least a polymerizable monomer, dispersing this monomer composition in an aqueous medium to form suspended particles and polymerizing the resultant suspended particles, wherein in the dispersing step, an initial viscosity of the monomer composition is controlled within a range from 1 to 100 mPa·s, and the resultant prepared liquid is then dispersed by means of a media type dispersing machine to control the viscosity after the dispersion within a range from 10 to 1,000 times as high as the initial viscosity.
In Examples of Japanese Patent Application Laid-Open No. 6-75429, a media type dispersing machine having a separation part for separating media particles is used as a media type dispersing machine. In this process, the viscosity of a polymerizable monomer composition greatly changes during the dispersing treatment, so that the colorant is hard to be uniformly dispersed, and further, the media particles are unevenly distributed in the separation part to lower dispersing efficiency. Therefore, this process has involved a problem that the dispersion level of the colorant becomes insufficient, or the time required for dispersion becomes long, and so production efficiency is greatly lowered.
U.S. Pat. No. 6,534,233 has proposed a production process of a polymerized toner, which comprises the dispersion step of dispersing a colorant in a dispersion medium containing a polymerizable monomer under conditions of a tip speed (peripheral speed) of an agitator (rotor) of 3 to 20 m/sec and a passing time of 0.03 to 0.5 hour by means of a media type dispersing machine to obtain a monomer composition. However, this process also cannot achieve a required dispersion level in the case of a color toner, of which a particularly high dispersion level is required, or takes a very long time to reach the dispersion level. Therefore, the process has involved a problem that production efficiency is worsened.